A secondary battery is a battery which can discharge the battery by converting a decrease in chemical energy accompanying chemical reaction to electrical energy, and store (charge) the battery by converting electrical energy to chemical energy by applying electrical current in a direction that is opposite to the discharge direction. Among secondary batteries, a lithium secondary battery has been widely applied for power sources for notebook personal computers, cellular phones, etc. since the energy density of the lithium secondary battery is high.
In a lithium secondary battery, if graphite (referred as to “C”) is used as a negative electrode active material, the reaction described by the following formula (I) proceeds at a negative electrode upon discharging the battery:LixC→C+xLi++xe−  (I)
wherein, 0<x<1.
Electrons generated by the reaction described by the formula (I) pass through an external circuit, work by an external load, and then reach a positive electrode. Lithium ions (Li+) generated by the reaction described by the formula (I) are transferred by electro-osmosis from the negative electrode side to the positive electrode side through an electrolyte sandwiched between the negative electrode and the positive electrode.
Also, if lithium cobalt oxide (Li1−xCoO2) is used as a positive electrode active material, upon discharging the battery, the reaction described by the following formula (II) proceeds at the positive electrode:Li1−xCoO2+xLi++xe−→LiCoO2  (II)
wherein, 0<x<1.
Upon charging the battery, reactions which are reverse to ones described by the above formulae (I) and (II) proceed at the negative and positive electrodes, thereby regenerating graphite into which lithium is inserted (LixC) by graphite intercalation at the negative electrode, and regenerating lithium cobalt oxide (Li1−xCoO2) at the positive electrode. Because of this, discharging becomes possible again.
A lithium secondary battery comprising a lithium solid electrolyte, a positive electrode active material and a negative electrode active material is produced as follows: a positive electrode active material layer, a lithium solid electrolyte layer and a negative electrode active material layer are laminated in this order to form a laminate, and the laminate is sintered by a heat treatment. By the above sintering, the positive electrode active material layer can adhere to the lithium solid electrolyte layer, and the lithium solid electrolyte layer can adhere to the negative electrode active material layer.
However, in the above-described method for bonding the interfaces by sintering, there has been a possibility that the sintered interface is electrochemically inactivated, the bonding of the interfaces is insufficient, and a side reaction, in which materials which are not contributed to discharging and charging are produced, proceeds at the interface between the active material and the solid electrolyte. Therefore, it is difficult to form an excellent interface between the active material and the solid electrolyte while densifying or crystallizing the active material layer and the solid electrolyte layer by a heat treatment.
As a technique for solving the problem as just described above, Patent Literature 1 discloses a laminate for an all-solid lithium secondary battery comprising an active material layer and a solid electrolyte layer which is bound together with the active material layer by sintering, wherein the active material layer contains the first kind of crystalline material capable of releasing and absorbing lithium ions, the solid electrolyte layer contains the second kind of lithium ion-conductive crystalline material, and when the laminate is analyzed by an X-ray diffraction method, components other than constituents of the active material layer and the solid electrolyte layer are not detected from the laminate.